1. Field of the Invention
The present invention relates to methods and apparatus for non-contact measurement of internal pressure changes in vessels with external surfaces deflection signatures that can act as pressure transducers. The methods are particularly suitable for physiological vessels or cavities.
2. Description of Related Art
Devices which measure blood pressure or the internal pressure of a physiological vessel or cavity are well known in the art. For example, devices used for measuring blood pressure in clinics or offices are generally known as sphygmomanometers, while those measuring fluid pressure within an eye are generally known as tonometers. The latter instruments measure the amount of tension on the eye's outer wall, allowing determination of fluid pressure within the eye. In order to measure outer wall tension, conventional tonometers often must have direct or indirect physical access to the outer wall to deform, displace or oscillate the outer wall. Analogously, measurement of blood pressure in most clinical situations requires application of an inflatable cuff to an arm or leg. In either case, special equipment is needed and, particularly in the case of tonometry measurements, the patient must visit a clinic or office for the measurement to be made.
Among the efforts to measure changes in intraocular pressure in the eye are methods such as that described by Eyed (1990) which work on photoelasticity principles. This involves temporary birefringence in the eye's lens from stresses due to intraocular pressure. A significant disadvantage of the photoelastic method is that the light source, and field before the receptor, must be polarized. For reasonable quantitative measure, quarter wave plates also must be placed between a light beam aimed at the eye's surface and between the reflection from the eye's external surface and the receptor. Photoelasticity works only with the transparent lens of the eye, which is the only part of the eye that generates birefringence and through which a light beam must pass to accumulate a signal. A further disadvantage of using photoelasticity methods is that in cases where the eye is being monitored in glaucoma patients, useful measurements are not made in advanced cases where birefringence becomes permanent and/or inconsistent with progression of the disease.